The objectives of this study are to characterize possible age-related differences in the in vitro responses of murine lymphohemopoietic cells to mitogens and their differential susceptibility to freezing damage. Current work focuses on the assessment of functional capacity in the in vitro environment where conventional culture methods may not accurately reflect true functional capacity. Augmented "responses" to all the tested mitogens were observed by raising H3-thymidine concentrations, reflecting what could be interpreted as increased DNA synthesis and cell proliferation. Enhancement of H3-thymidine incorporation was observed in different strains of mice and in both young and old mice.